In the context of the present application, the term steam turbine is to be understood as meaning any turbine or part-turbine through which a working medium in the form of steam flows. By contrast, gas turbines have gas and/or air flowing through them as working medium, but this medium is subject to completely different temperature and pressure conditions than the steam in a steam turbine. Unlike in gas turbines, in steam turbines the working medium flowing to a part-turbine, for example, reaches its highest pressure at the same time as it is at its highest temperature. Therefore, an open cooling system, as in gas turbines, cannot be realized without a supply from the outside of the part-turbine.
A steam turbine usually comprises a rotor which is fitted with blades, is mounted rotatably and is arranged inside a casing shell. When heated and pressurized steam flows through the interior of the flow space formed by the casing shell, the rotor is made to rotate by the steam via the blades. The blades of the rotor are also known as rotor blades. Furthermore, stationary guide vanes which penetrate into the spaces between the rotor blades are usually attached to the casing shell. A guide vane is usually held along an inner side of the steam turbine casing at a first location. In this form, it is usually part of a ring of guide vanes which comprises a number of guide vanes which are arranged along an inner circumference on the inner side of the steam turbine casing. The main vane part of each guide vane faces radially inward. A ring of guide vanes at the above mentioned first location along the axial extent is also referred to as a row of guide vanes. A number of rows of guide vanes are usually positioned one behind the other. Accordingly, a further, second vane is held along the inner side of the steam turbine casing at a second location behind the first location along the axial extent.
The casing shell of a steam turbine of this type may be formed from a number of casing segments. The casing shell of the steam turbine is to be understood as meaning in particular the stationary casing component of a steam turbine or part-turbine which, along the axial extent of the steam turbine, has an inner space which is provided for the working medium steam to flow through. Depending on the type of steam turbine, this may be an inner casing and/or a guide vane carrier. However, it is also possible to provide a turbine casing which does not have an inner casing or a guide vane carrier.
For efficiency reasons, the design of a steam turbine of this type for what are known as “high steam parameters”, i.e. in particular high steam pressures and/or high steam temperatures, may be desirable. However, for materials science reasons, it is not possible in particular to increase the temperature without restriction. To allow the steam turbine to operate reliably even at particularly high temperatures, the cooling of individual parts or components may be desirable.
With the coolant methods which have been disclosed hitherto, in particular for a steam turbine casing, a distinction has to be drawn between active cooling and passive cooling. In the case of active cooling, cooling is brought about by a cooling medium which is fed to the steam turbine casing separately, i.e. in addition to the working medium. By contrast, passive cooling is brought about only by suitably guiding or using the working medium. Standard cooling of a steam turbine casing is restricted to passive cooling. For example, it is known for cool, ready-expanded steam to flow around an inner casing of a steam turbine. However, this has the drawback that a temperature difference across the inner casing wall has to remain restricted, since otherwise the inner casing would be excessively thermally deformed in the event of an excessively high temperature difference. Although heat is dissipated with flow around the inner casing, the dissipation of heat takes place relatively far away from the location where it is supplied. Hitherto, it has not been possible to achieve sufficient dissipation of heat in the immediate vicinity of where the heat is supplied. Further, passive cooling can be achieved by suitable implementation of the expansion of the working medium in what is known as a diagonal stage. However, this only makes it possible to achieve a very limited cooling action on the casing.
U.S. Pat. No. 6,102,654 describes active cooling of individual components inside a steam turbine casing, the cooling being restricted to the inflow region of the hot working medium. As shown in FIG. 1 of this application, according to U.S. Pat. No. 6,102,654 cooling medium is passed through the casing onto a protective shield and onto a first ring of guide vanes, in order to reduce the thermal load on the rotor and the first ring of guide vanes. Some of the cooling medium is admixed with the working medium. The cooling is in this case supposed to be brought about by flow onto the components which are to be cooled.
It is known from WO 97/49901 for a single ring of guide vanes to be acted on selectively by a medium, through a separate, radial channel in the rotor which is supplied from a central cavity, in order to shield individual regions of the rotor. For this purpose, the medium is admixed with the working medium via the channel, and cooling medium flows selectively onto the ring of guide vanes. However, with the central hollow bore of the tube which is provided for this purpose, it is necessary to accept increased centrifugal force stresses, which constitutes a significant drawback in terms of design and operation.
EP 1154123 has described a possible way of removing and guiding a cooling medium from other regions of a steam system and the supply of the cooling medium in the inflow region of the working medium.
To achieve higher efficiency levels in the generation of power using fossil fuels, there is a need to employ higher steam parameters, i.e. higher pressures and temperatures, than has hitherto been customary in a turbine. In this context, if steam is used as the working medium, pressures of in some cases well over 200 bar and temperatures of in some cases well over 500° C. are intended. Steam parameters of this nature are described in detail in the article “Neue Dampfturbinen-konzepte für höhere Eintrittsparameter und längere Endschaufeln” [Novel steam turbine concepts for higher entry parameters and longer end blades] by H. G. Neft and G. Franconville in the Journal VGB Kraftwerks-technik, No. 73 (1993), Volume 5. The content of disclosure of this article is hereby incorporated by reference in the description of the present application. In particular, examples of higher steam parameters are cited in FIG. 13 of the article. In the abovementioned article, a cooling steam supply and passage of the cooling steam through the first row of guide vanes and if appropriate also through the second row of guide vanes is proposed in order to improve the cooling of a steam turbine casing. Although this provides active cooling, it is restricted to the main flow region of the working medium and is still in need of improvement.
Therefore, all the methods which have been disclosed hitherto for cooling a steam turbine casing, if they are active cooling methods at all, at best provide for a directed flow onto a separate turbine part which is to be cooled and are restricted to the inflow region of the working medium, at most including the first ring of guide vanes. When higher steam parameters are applied to standard steam turbines, an increased thermal load may result over the entire turbine, and this load could only be alleviated to an insufficient degree by standard cooling of the casing as described above. Steam turbines which fundamentally use higher steam parameters in order to achieve higher efficiencies require improved cooling, in particular of the casing, in order to sufficiently break down the higher thermal load on the steam turbine. This gives rise to the problem that when turbine materials which have hitherto been customary are employed, the increasing load on the casing resulting from increased steam parameters may lead to a disadvantageous thermal load on the casing, and consequently these may no longer be technically feasible.